The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as carotid arteries, coronary arteries, peripheral arteries, veins, or other vessels to maintain the patency of the lumen. More particularly, the invention relates to the design and configuration of the geometry of stent struts so as to minimize the disturbance to the blood flow in the vessel, and to minimize the trauma caused by the stent to the body lumen in which it is implanted.
Stents are frequently used in the treatment of atherosclerotic stenosis in blood vessels especially in conjunction with percutaneous transluminal angioplasty (PTA) or percutaneous transluminal coronary angioplasty (PTCA) procedures, with the intent to reduce the likelihood of restenosis of a vessel. Stents are also used to support a body lumen, tack-up a flap or dissection in a vessel, or in general where the lumen is weak to add support. Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as a coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use in supporting and holding back a dissected arterial lining which can occlude the fluid passageway there through.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency and are especially well-suited for the treatment of atherosclerotic stenosis in blood vessels. Intracoronary stents have become a standard adjunct to percutaneous coronary angioplasty in the treatment of arterial atherosclerotic disease. Although commercial stents vary in design and materials, they share similar structural features. Most stents in clinical use are metallic and are either self-expanding or are expanded by the force of an expandable member, such as an angioplasty dilatation balloon. These devices are typically implanted via a delivery catheter which is inserted at an easily accessible location on the patient and then advanced through the patient""s vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen and into the stenosis. Once in position, the stent is deployed which, depending upon its construction, is achieved either automatically by the removal of a restraint, or actively by the inflation of a balloon about which the stent is carried on the delivery catheter.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First and foremost, the stent must be capable of withstanding the structural loads that are imposed thereon as it supports the lumen walls. In addition to having adequate radial strength or more accurately, hoop strength, the stent should nonetheless be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material of which the stent is constructed must allow the stent to undergo expansion, which typically requires substantial deformation of localized portions of the stent""s structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear upon it, including the cyclic loading induced by the pulsatile character of arterial blood flow. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses. A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter, helically wound coiled springs manufactured from an expandable heat sensitive metal, and self-expanding stents inserted into a compressed state for deployment into a body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from, for example, shape memory metals or super-elastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the body lumen. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent.
Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maass, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallsten); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam). Further details of prior art self-expanding stents can be found in U.S. Pat. No. 4,580,568 (Gianturco); and U.S. Pat. No. 4,830,003 (Wolff, et al.).
Despite the widespread use of intracoronary stents, in-stent restenosis remains a major clinical problem; however, restenosis does not develop in all patients undergoing coronary angioplasty and stent implantation. The mechanism of restenosis after stent implantation is principally neointimal hyperplasia, as stents resist negative arterial remodeling. Relative to PTCA alone, stents improve the outcome by minimizing vessel recoil, reducing plaque prolapse, and affecting long term remodeling.
It has been shown that smooth muscle cell (SMC) proliferation is affected by signaling from the endothelium. In flowing blood, the endothelium seeks a surface shear stress of approximately fifteen dynes/cm2. This tendency is known as the xe2x80x9cGlagov Effect.xe2x80x9d If the shear stress is too high, the endothelial cells signal the smooth muscle cells to relax. When the detected shear stress is low, the endothelium signals for the vessel to constrict. If the shear stress remains consistently low, SMC proliferation occurs causing lumenal narrowing. Regions of flow reversal, or disturbed blood flow, also cause the endothelial cells to signal for lumenal narrowing in an effort to maintain the desired shear stress. This is one reason why atherosclerotic legions form first at vessel bifurcations and other regions of complex flow. Another reason is that in these regions there is an increased residence time of blood elements including atherogens such as lipids and cholesterol which increases the chance of deposition. It is known that stent struts alter the blood flow. Immediately upstream and downstream of the struts, the flow is disturbed, with flow reversals and eddies. Thus, the introduction of the stent into the vessel can cause lumenal narrowing and potentially sets the stage for further atherosclerotic disease.
Animal studies also have established a significant correlation between the degree of arterial injury caused by metallic stents and the resultant neointimal thickness and lumen stenosis at the stented site. Patients with restenosis may be those whose vessel incurred greater injury during revascularization. Indeed, breakage of the internal elastic lamina (IEL) has been correlated with a higher level of restenosis. Similarly, higher injury score has been implicated as a causal factor in neointimal formation, resulting in restenosis. In addition, inflammation caused by the implantation of the stent may cause neointimal growth, and a deeper arterial laceration causes a greater inflammatory reaction. Thus, the amount of neointimal formation may be proportional to the degree of vessel injury, the extent of inflammatory reaction, or both, independently or in combination, wherein the deep arterial lacerations cause the thickest neointima leading to a more severely compromised lumen. Stent design, its geometric configuration, and the amount of surface metal coverage also affects the degree of vessel wall injury, the resulting inflammatory response, and the quality of neointima formed after implantation of the stent.
What has been needed, and heretofore unavailable, in the art of stent design is a geometric configuration of the stent struts which minimizes neointimal growth. Such a design should reduce the disturbance of the blood flow within the vessel. Moreover the optimal stent geometry should reduce the injury and inflammation of the vessel wall. The present invention satisfies these and other needs.
Briefly, and in general terms, the present invention is directed to the design and configuration of stents that will minimize the disturbance of blood flow in the vessel and that will minimize the trauma caused by the stent to the vessel wall in which it is implanted. The stent of the present invention includes struts having an outer surface configured with a geometry that contacts a vessel wall so as to reduce the injury, and inflammation, to the several layers of the vessel wall. In addition, the stent struts have an inner surface that contacts the blood flow, wherein the inner surface is configured with a geometry that reduces the turbulence of the blood flow as it passes over the struts. Moreover, the shape of the outer surface may be different from the shape of the inner surface. Such a configuration reduces the disturbance to the endothelium created by changes in fluid shear stress, and minimizes trauma to the vessel wall, leading to a decrease in neointimal hyperplasia.
Despite the widespread use of intracoronary stents, in-stent restenosis remains a major clinical problem. Restenosis after stent implantation is principally caused by neointimal hyperplasia. It has been shown that smooth muscle cell proliferation is affected by signaling from the endothelium for lumenal narrowing. Immediately upstream and downstream of the struts, the flow is disturbed, with flow reversals and eddies. In addition, breakage of the internal elastic lamina and inflammation caused by the implantation of the stent has been correlated with a higher level of restenosis. Stent design, specifically the geometric configuration of the stent struts, can affect the amount of vessel wall injury and inflammation, as well as the shear stress and turbulence of the blood flow over the stent. Thus, the strut geometry may effect neointimal formation after implantation of the stent.
The stent of the present invention has a novel geometric configuration of the outer surface and the inner surface of the stent struts. One particular shape of the outer surface of the stent strut is one that distributes the stress evenly over the stent outer surface. In order to provide good vessel scaffolding, while only covering the minimal amount of vessel wall area, stent struts have a high aspect ratio being longer than they are wide. Starting with the stent structure, the present invention describes outer and inner strut surfaces. The geometry of these surfaces can be largely defined by a cross-section, as struts are typically constant in cross-section along their length. This invention discloses particular stent cross-sections, which minimize the stress applied to the vessel wall and the disturbance to the blood flow. Many orientations relative to the strut itself are possible for this strut cross-section. The stent cross-section referred to herein lies in the plane that intersects the central axis of the vessel. In this discussion, the vessel is cylindrical and straight. This plane is coincident with the direction of blood flow and lies perpendicular to the hoop stress that a stent exerts on the vessel. Within a deployed stent, the struts may lie in a variety of orientations relative to the vessel axis. In all cases, this optimum strut cross-sectional geometry is defined along this plane that is, of course, parallel to and intersects the vessel axis. The shape of the outer surface of the stent that distributes the stress imposed on the inner elastic lamina evenly over the stent outer surface is a singular circular arc. In one aspect of the present invention, the arc is one whose radii of curvature is such that at the strut edge, the stent surface slope matches that of the vessel wall. The radius of curvature (r) of the strut outer surface can be calculated as a function of the strut width (W) and the angle (xcex8) with which the vessel wall intercepts the stent strut. The radius of curvature can then be defined as: r=W÷(2sin xcex8). In this configuration, the outer surface is symmetric relative to a radial vector, this vector being defined as running from the vessel axis outwards towards the vessel wall.
An optimal geometry of the stent inner surface is one in which the shape disturbs the blood flow the least. To minimize the disturbance of blood flow, the inner surface should provide a smooth transition at the interface between the strut leading edge and the vessel wall. Likewise, there should be a smooth transition at the interface between the strut trailing edge and the vessel wall. However, the shape of the ideal inner surface is also a function of the strut width and the amount of strut cross-sectional area needed for strength, and may not be symmetric with respect to a radial vector. Providing strength to the stent, while providing hydrodynamics that minimizes the disturbance to the blood flow, yields a wing-like (foil) shape to the inner surface of the strut. Thus, the overall optimal geometry of the stent strut of the present invention combines an outer surface that minimizes vessel injury with an inner surface whose hydrodynamics minimizes the disturbance to the blood flow in the vessel.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the invention.